A balun transforms power between differential (balanced) and single ended (unbalanced) circuits or conversely between single ended and differential circuits, and is commonly used in radio frequency (RF) and microwave frequency circuits such as mixers, push pull amplifiers, and frequency doublers. A Marchand balun is a passive balun configured as a transmission line type of transformer often used in microwave applications, which provides both a wide bandwidth and DC isolation from primary to secondary. In the past, Marchand baluns and particularly planar topologies have played an important role in the development of several different circuit applications such as mixers, frequency doublers and push-pull amplifiers by providing a wideband conversion capability from an unbalanced input to balanced outputs. Traditionally, active baluns have been widely employed in monolithic microwave integrated circuit (MMIC) applications; however, they do compromise the system's dynamic range by generating extra noise.
Passive MMIC components have been loaded with an increasing variety of planar Marchand baluns ranging from purely lumped solutions to lumped-distributed and fully distributed circuits. Although the latter ones show the greatest bandwidth performance, the requirement of two quarter wave-length sections made the baluns more suitable for the higher frequency bands of the radio frequency (RF) spectrum in order to lower fabrication costs. The latest such use of planar Marchand baluns resulting from advances in MMIC technology processes and materials have allowed for profitable fully distributed planar Marchand balun circuit implementations at frequencies as low as 1 GHz and can provide for at least one to four octaves of bandwidth. Several exemplary balun devices are discussed hereinbelow.
U.S. Pat. No. 5,061,910 entitled BALUN TRANSFORMERS teaches a balun transformer formed by printed tracks on a printed circuit board. The balanced side of the balun is entirely on one side of the board resulting in symmetrical parasitic effects in the balanced limbs. However, there is no mention about implementing the coupled sections in an asymmetrical manner in order to overcome these parasitic effects.
U.S. Pat. No. 6,683,510 entitled ULTRA-WIDEBAND PLANAR COUPLED SPIRAL BALUN teaches a coupled transmission line balun construction employing two pairs of planar interleaved spiral coils formed on an electrically insulating or semi-insulating substrate defining a planar structure, and having a shunted capacitor joining the two coils of the secondary. The balun provides an ultra-wide bandwidth characteristic for MMIC devices. However, this balun's performance is limited by the symmetrical structure of the coupled sections.
U.S. Pat. No. 7,250,828 entitled COMPACT BALUN teaches a distributed backwards-wave balun for use in wireless, cellular handsets and radios and in RF modules. The input to the balun is presented on the inner side of the first coupled section. However, this balun's performance is limited by the symmetrical structure of the coupled sections.
U.S. Pat. No. 7,605,672 entitled INVERTED STYLE BALUN WITH DC ISOLATED DIFFERENTIAL PORTS teaches a balun including a first coupler structure having a first port of a balanced port pair and an unbalanced port, and a second coupler structure including a second port of the balanced port pair. The second coupler port structure is connected to the first coupler structure such that the second port of the balanced port pair is DC isolated from the first port of the balanced port pair without decoupling components. However, this balun's performance is limited by the symmetrical structure of the coupled sections.
U.S. Pat. No. 7,772,941 entitled ULTRA-WIDEBAND/DUALBAND BROADSIDE-COUPLED COPLANAR STRIPLINE BALUN teaches a balun utilizing asymmetric coplanar striplines. Although this balun has been implemented with asymmetrical sections, those are coplanar stripline asymmetric and nonuniform, relying on broadside coupling phenomena realized exclusively with straight lines.
U.S. Pat. No. 7,880,557 entitled HYBRID MARCHAND/BACK-WAVE BALUN AND DOUBLE-BALANCED MIXER USING SAME teaches a hybrid Marchand/back-wave balun and a double balanced mixer using the hybrid balun. This hybrid (lumped/distributed) balun has the disadvantage of switching between topologies at different frequencies, which is problematic for matching and phase control.
U.S. Patent Publication Number 2007/0120621 entitled VERTICAL INTER-DIGITAL COUPLER teaches a vertical inter-digital coupler employed in radio-frequency and/or microwave components, and particularly for RF and/or microwave coupled transmission line components. However, this coupler's performance is limited by the symmetrical structure of the coupled sections.
While design optimization for homogeneous transverse electromagnetic (TEM) lines is established, when inhomogeneities in the dielectric occur, the methodology is no longer valid. Such inhomogeneities occur when the electromagnetic wave carried by a microstrip line exists partly in the dielectric substrate, and partly in the air above it. In general, the dielectric constant of the substrate will be different (and greater) than that of the air, so that the wave is travelling in an inhomogeneous medium. In consequence, the propagation velocity is somewhere between the speed of radio waves in the substrate, and the speed of radio waves in air. Thus, a new theory for the design of asymmetrical coupled line directional couplers in inhomogeneous media has been developed, and is discussed in the paper Aaron Vaisman, “A generalized S-Parameter Analysis of Novel Planar Marchand Balun Topologies” IEEE Wamicon Conference, Invited Paper, 2013 which is herein incorporated by reference in its entirety. The practical application of this theory as discussed hereinbelow results in the present invention.
A higher-performing balun would be realized in the form of monolithic microwave integrated circuits with asymmetric uniform coupled line sections embedded, designed specifically for use in inhomogeneous media, and optimized by applying a mathematical scattering parameter analysis providing for proper output matching and concomitant better phase unbalance performance.